The stats page lists the 15 states with the highest number of declared LiveJournal users. For each state, here is the number of declared LiveJournal users in that state divided by the population of that state as of April 2000, sorted in order of largest ratio to smallest ratio. I did this because I suspected that Massachusetts would be the highest, and indeed it is.

At least in the Cambridge/Somerville area, there seems to be a lot of overlapping social circles, so, once it got a foothold, it spread like the plague. Also, there are many, many colleges in Boston and surrounding areas, so I imagine that might have something to do with it, too.